In cases where full color prints are conducted by printing, thermo-transfer, inkjet, and electrophotography, etc., generally used are four colors of MMCK as outputting colors of the color prints. Conventionally various methods are tried as the method of expressing a color by the use of four colors in calorimetric way.
For example there is a first method in which predetermined weighting is applied to a K amount of a color solid made by the condition of Y=0, M=0, and C=0 to determine a new K amount, and based on the value of the K amount, new Y, M, C amounts are determined in calorimetric way (Ninth Color Engineering Conference Collected Papers, Flexible URC using L*a*b*, the first report: an idea of new UCR capable to be applied on both character and halftone, by Shinji Kita, Sei Kokatsu, 1992).
As the second method, there is a method in which a combination of CMYK to reproduce a target color is obtained from among four color solids determined by the conditions of Y=0, M=0, C=0, and K=max (Refer to Unexamined Japanese Patent Application Publication No. HEI2-136848). By this method, total ink amount can be reduced by maximally using K amount as well as ensuring the maximum color gamut which is capable of being reproduced by four color printers.
As the third method, there is a method in which a combination of CMYK to reproduce a target color is obtained from among four color solids determined by the conditions of Y=max, M=max, C=max, and K=0 (Refer to Japanese Patent Application No. HEI4-266718). In this method K amount is minimally used and the total ink amount increases, though, noises are suppressed by the increase of used colors and a gradation is improved.
However, according to these various conventional methods, since positions are generated where K amount varies rapidly with in accordance with changes of the target colors, a common problem of generating false contours arises according to variations in printers. Although, an identical target color can be theoretically reproduced by combinations of YMCK with different K amounts, in actual, since there are variations in printers, errors may be generated in combinations of K amounts with large difference. Even in cases where the increasing/decreasing direction of brightness as the example of prescribed color element of the target color (which corresponds to density variation direction of the reproduced color) and the direction of increasing/decreasing direction of K amount are set in agreement with each other, non-smoothness may arise to cause the false contours in an rising area from no-use region to use region of K and in a boundary area where K amount is fixed to be maximum after going through smooth variation area.
Further, in cases where a combination of a function conversion by LUT (Look Up Table) and interpolation by an interpolation device is utilized for the conversion into image signals (CMYK) for reproducing the image signals (R, G, B, etc.) displayed on a target device such as CRT by a printer as a reproducing device, if the interpolation is executed based on the data of color cubic of both sides of the non-smoothness area, the interpolation error may become large, since between the data are non-smoothly connected with broken lines.
Further, in the first method, since among the combinations made by the condition of Y=0, M=0, and C=0, the K amount is calculated to reproduce the target color, there exists an area that cannot be inherently used among the maximum color gamut reproducible by a four color printer. Namely, when K amount is in maximum, up to two colors of YMC can be maximally used, but three colors cannot be used at a time.
Further, with respect to K amount to reproduce the target color, namely with respect to the maximum K amount obtained in the condition of Y=0, M=0, and C=0, if the ratio of K amount is arbitrarily determined, in case of the color near the boundary area of reproducible color gamut when K amount is decreased and replaced by the other three colors, for example in a condition of Y=0, K amount is 90 (max=100), and M, C>60, if K amount is decreased to be a half value 45, the amounts of Y, M, C is needed to be increased for compensation of decreased K amount and the amounts of M, C may exceed 100 to be outside the gamut, this disables the correct color reproduction.
On the other hand, as described above, in the third method K amount is utilized in minimum in order to increase the gradation, however, in actual there are cases where increased gradation is preferable and where decreased gradation is preferable. In the second or the third methods, by the restriction of utilizing the maximum or the minimum K amounts, there is no flexibility of adjusting the ratio of K amount according to colors in order to obtain a preferable image quality.
Regarding these method, Patent document 1 and Patent Document 2 described below are proposed.
Patent Document 1: Examined Japanese Patent Application Publication No. 3273204 (1st page, FIG. 1)
Patent Document 2: Unexamined Japanese Patent Application Publication No. 2001-169131 (1st page, FIG. 1)